The present invention relates to a solution and process for reducing or minimizing surface oxidation of a metal deposit provided by a plating process such as electroplating. The solutions and processes also provide improved deposit properties including appearance and solderability.
Electroplated tin and tin alloy coatings have been used in electronics and other applications such as wire, and continued steel strip for many years. In electronics, they have been used as a solderable and corrosion resistive surface finish for contacts and connectors. They are also used a lead finish for integrated circuit (“IC”) fabrication. In addition, a thin layer of tin or tin alloy is applied as the final step for passive components such as capacitors and transistors.
Though applications vary, there are some commonalities regarding the requirements for this final surface finish. One issue is long term solderability, defined as the ability of the surface finish to melt and make a good solder joint to other components without defects that would impair the electrical or mechanical connection.
There are many factors that determine good solderability, the three most important of which are extent of surface oxide formation, amount of codeposited carbon, and extent of intermetallic compound formation. Surface oxide formation is a natural occurring process because it is thermodynamically favorable. The rate of formation of the surface oxide depends on the temperature and time. In another words, the higher the temperature and longer the time, the thicker the surface oxide that is formed. In the case of electroplated tin or tin alloy coatings or deposits, surface oxide formation also depends on the surface morphology of the coating or deposit. When comparing pure tin to tin alloy coatings, for example, tin alloys generally form less or thinner surface oxides when all other conditions are equal.
Codeposited carbon is determined by the plating chemistry one chooses to use. Bright finishes contain higher carbon contents than matte finishes. Matte finishes are normally rougher than the bright finishes, and provide an increased surface area that results in the formation of more surface oxides than typically are formed with a bright finish. The plater thus has a trade off between potential amount of surface oxide and surface finish.
Intermetallic compound formation is a chemical reaction between the tin or tin alloy coating and the substrate. The rate of formation depends on temperature and time as well. Higher temperatures and longer times result in a thicker layer of intermetallic compounds.
To improve or ensure the highest degree of solderability, it is important to 1) use a non-bright tin or tin alloy plating solution, 2) deposit a sufficient layer of tin or tin alloy so that surface oxide or intermetallic compound formation will not consume the entire layer, and 3) to prevent or minimize exposure of the tin plated surface to elevated temperatures for extend periods of time.
It is relatively easy to achieve 1) and 2), but it is very difficult to achieve 3). The temperature and time of subsequent part treatment after plating of a tin or tin alloy deposit is normally dictated by the assembly specifications and existing manufacturing layout and practice. For example, in “two tone” leadframe technology, after the tin or tin alloy plating, the entire package will have to go through many process steps (i.e., a long period of time for such treatments) which require multiple thermal excursions at temperatures as high as 175° C. Inevitably, more and/or thicker surface oxides form, and this in turn reduces the solderability of the tin or tin alloy deposit. In current processing, it is not possible to omit these additional steps since the final components or assemblies will not be complete.
Therefore it is highly desirable to find ways to prevent or minimize surface oxide formation on such parts. One known way to do this is to introduce a conformal coating on the surface of the tin or tin alloy deposit. This technology can be summarized in two general categories: one that applies a precious metal coating and the other that applies an organic coating. The first category is undesirable for protection of tin or tin alloy deposits because it introduces an expensive, extra process step. The second category is also undesirable because it will inevitably introduce impurities onto other critical areas of the leadframe or electrical component due to the non-selective nature of the organic coating that is deposited. These impurities have proven to be detrimental to the subsequent leadframe and IC assembly processes.
Accordingly, further solutions to this problem are needed, and these are now provided by the present invention.